Consequences of Grafting Density on Bottlebrush Rheology

The linear viscoelastic behavior of poly(norbornene dimethyl ester)-g-poly(DL-lactide) graft polymers was investigated as a function of grafting density, ranging from 0 to 100 percent, and backbone molecular weight at constant graft chain molecular weight. Master curves reveal that the zero-shear viscosity of these polymers display a sharp transition from Rouse to reptation scaling, demonstrating that grafting density strongly impacts the entanglement molecular mass. The scaling of the entanglement plateau modulus (G_e) as a function of the grafting density was compared to theoretical predictions. Polymers with high and low grafting density were found to be consistent with theoretical models for dense brushes (G_e ~ n_g^1.2) and loose combs (G_e ~ n_g⁰). Graft polymers with intermediate grafting densities did not fit existing models for graft polymers (G_e ~ n_g^3.5), including loose brushes and dense combs. The strong dependency of G_e in the intermediate regime reflects an increase in flexibility, which is attributed to backbone “kinking” to space out the side chains as grafting density increases.